Helmets in sports, military and law enforcement are generally composed of an outer shell and an inner liner. Depending on the intended use (impact vs. ballistic) the shell can be made of a thermoplastic, ABS, polycarbonate, carbon fiber, Kevlar, or any number of other materials known to those skilled in the art. Liners evolved from a simple strap or web suspension system meant to separate the wearer's head from the shell to designs utilizing fluid displacement, air, and/or crushable foam in one or more layers tailored to respond to varying impact strengths.
With respect to head impacts and trauma, the most common injuries result from direct impacts resulting in linear acceleration of the head, and tangential blows resulting in angular or rotational acceleration. The energy transfer from a direct impact may result in either or both linear and rotational acceleration of the brain inside the skull. The consistency of brain material is similar to gelatin and once accelerated inside the skull the brain impacts the side of the skull opposite the impact. The point of impact is called the “coup,” and the “slap” injury from the brain impacting the opposite side of the skull is called the “contra-coup injury.” Bleeding and tissue damage frequently occur at both sides with adequate trauma. Furthermore, the inner surface of the skull has many depressions and ridges of bone. Some areas are particularly rough, and as the brain travels over these surfaces additional damage and bleeding can occur. Underdeveloped brains of adolescents incur damages that may not show up until later in adulthood. The human brain does not fully develop until well in the 20's.
Tangential impacts impart a rotational acceleration to the brain. Different types of brain material have slightly different density, and as such experience greater or lesser rotational momentum under such circumstances. This difference in momentum results in shear forces that tear the brain matter, and more specifically, axons. In severe cases, this results in diffuse axonal injury. Shear injuries are also common at points of relative attachment/securement or immobility within the brain, such as the corpus callosum.
Varying degrees of these patterns of injury account for the dramatic number of concussive and sub concussive injuries observed in professional, collegiate, and youth sports. These injuries can result in life-threatening consequences; however, more frequently result in residual cognitive and behavior deficits. Ongoing exposure can result in temporary loss of brain function leading to cognitive, physical and emotional symptoms, such as confusion, vomiting, headache, nausea, depression, disturbed sleep, moodiness, and amnesia leading to Chronic Traumatic Encephalopathy (“CTE”), a neurodegenerative condition with similarities to Alzheimer's Disease.
To this point, the helmet shell has done little more than prevent penetrating trauma and skull fractures, which is what it was originally designed for and continues to perform. Much work has been performed on the liner systems to attempt to mitigate energy transfer to the wearer. Current liner systems employed have a maximal extent of compressibility after which their ability to mitigate energy transfer is markedly diminished. These liner systems are best suited to direct impacts where the compressibility of the foam or fluid systems mitigates a portion of the energy transfer to the wearer. However, in the setting of more tangential impacts resulting in rotational acceleration, these methods offer little to prevent injury. At the time of impact the shell experiences an immediate rotational acceleration, which is directly imparted to the liner system, as they are mechanically connected. Current liner systems are optimally suited to dissipate energy in a plane perpendicular to the shell and liner surface offering little in the form of energy dissipation and attenuation prior to rotational energy transfer to the wearer.